Erratic synchronization on Term/Entry Number in Multiterm 2017

Synchronization on Term or on Entry Number works erratically in Multiterm 2017. With certain databases it works, with other it doesn't. Log message is "Does not conform to the target termbase definition".

However, I am doing tests by exporting all entries (default export) and reimporting them all on term/entry number, so the termbase definition is the same for the source and the target database.